You can have multiple OBi units in your home, yes.
No, you cannot share the same address.
However, I think you've left something out. With a modem connected to the internet, you also need a router. That may be built into your unit, or it may be standalone. The router takes the ONE address given to you by your cable/DSL company and gives each device in your home an individual address.
Each OBi device must have a unique address within your home network, just like every other device attached to that network (for example, each computer you have has a unique address).
If you have a router, then you can have several OBi devices.